Darn, that is a nice page. I sincerely wish I had your skill with web design.

Take off the features button. You'll be stealing a metric ton of clicks from download. Additionally, put a big bloody download button on the features page, too.

At the front page, you have a download text link at the bottom. Replace or augment this with another big bloody download button. It will make a difference, trust me.

There is no law that says you can't bold the important bits of testimonials. Especially when they're light grey, they don't exactly catch the eye like blah blah blah NATURALLY, EASILY, and QUICKLY blah blah blah BEST MONEY I EVER SPENT blah blah blah does.

Your testimonials page is too long and too undirected. I'd suggest aligning it like you do on your features page -- this will make it seem like you have a lot of testimonials (you do) and eliminate scrolling (always a plus). Then, and you will hear me repeat this theme a few more times, put a big bloody download button on this page. Otherwise the page is like a roach motel -- users check in, they don't check out. (Why make someone who just got done ready testimonial #471 have to scroll up to the top of the page, locate the main navigation bar, figure out which one is the download, click to that page, then click the download button? Big bloody download button! One click to happy! Attention span of gnat on meth!)

The individual features on your features page don't lead anywhere. Every last one of those pages is in your conversion funnel. You know what they would be improved by? That's right, a big bloody download button. Also, forward/back navigation might be a plus, but only after you add the big bloody download button.

Incidentally, put more text on all of them and you'll get some serious search engine loving.

With just a little more text that page could be optimized to rank well for things like "software metronome", etc. Assume somebody reading it has NOT read the other things on your site (i.e. start with a line like "The Cantabile audio performance software includes a metronome. This allows you to...")

Now, look at what I wrote above regarding Big Bloody Download Buttons -- it all goes ditto for Big Bloody Buy Buttons.

Your purchasing page says "Click on feature descriptions for more information." This is not a good idea -- the purchasing page should make a last, focused attempt on grabbing the sale from someone who has spent serious effort evaluating your software, not on sending them down a rabbit hole of a million and one things to investigate which do not include "put in your credit cards". Put two to three high-impact testimonials prominently on this page, using the bolding trick I already discussed.

Put credit card logos on or near your purchase buttons. You will not believe how much this increases transactions -- test if you don't believe me. Paypal gives buttons you can use for this purpose -- they've been tested exhaustively because they make Paypal millions. (See my purchasing page for examples.)

Make the moneyback guarantee more prominent. (I'd suggest making it an icon.

I really liked the idea of how screen shots are displayed. This is something quite interesting. The only thing I'd do is to make sure that when you click on the header, the buttons on the bottom don't jump about. Perhaps, putting the top buttons on the side like a list and save space to show the picture and text, but making sure that layout is steady. I think your idea is way better than jumping popups with zoom and faded background. Cool stuff. Good luck!